My main requirement for such a shield was to have Arduino-form factor, so that I can just plug it into the Arduino, with no extra work required. Ideally, it would also have an on-board (attached) battery of at least 1000mAh.

Considering that the height of the headers in an Arduino shield is about 8mm and the space between them is about 43mm, the dimensions of the battery should not exceed 50mm x 43mm x 8mm.

None of the three solutions presented here met this requirement, at least not without modifications.

1. 5V DC-DC step-up power pack

This "power pack" shield comes with an on-board (attached) 1500mAh Li-Poly battery. The battery is wider than 43mm, so the headers are not (and cannot be) present. Therefore, the shield is not plug-able, which would technically disqualify it as an Arduino shield.

The "power pack" shield has the Arduino form factor though, with the matching 3 mounting holes, so it can be attached to the underside of an Arduino. (Note: Wiseduino's mounting holes are not aligned with those on Arduino 2009/Uno).
A schematic is not provided.

This battery shield does not come with a battery, but it has 6 big mounting holes that "can help you easily fix the battery pack". I could not find any photo (for inspiration) of the shield with batteries attached.

As shown in the photo above, the battery shield does not have the expansion female headers. It can be plugged into the Arduino, but no other shield plugged on top of it.

3. Solar charger shield V0.9

Seeedstudio discontinued this shield and replaced it with "Solar charger V2.0" (they also have a cheaper, non-shield version, the "Li-Po Rider").

This charger shield uses CN3083 chip. Schematic is published here and documentation can be found here.

The shield does not come with a battery, but it has the expansion headers installed. As expected, the ubiquitous, 68mm x 48mm flat Li-Ion 2000mAh battery (pictured connected to the shield in the photo above) does not fit between the headers, so stacking this shield only works with a smaller (both capacity and dimension-wise) type of battery, probably this 1000mAh one (50mm x 35mm x 5mm).

This battery shield would be the closest to my requirement, only if it had a battery.

Other battery chargers, shields or not, that I did not investigate yet are: